A typical gas turbine is constituted of a compressor, a combustor, and a turbine, and air taken in from an air intake port is compressed by the compressor into high-temperature, high-pressure compressed air. The compressed air is supplied to the combustor, and, in the combustor, high-temperature, high-pressure combustion gas is generated by supplying fuel to the compressed air and combusting it. Since the combustion gas drives the turbine connected to the compressor, for example, power can be generated at the generator driven by the gas turbine by connecting the generator to the output shaft of the gas turbine.
With such a gas turbine, an active clearance control (hereinafter referred to as ACC) system carries out control to minimize the tip clearance, which fluctuates in response to the effect of temperature and centrifugal force, which vary in accordance with the operating state, preventing interference between rotary parts and stationary parts and achieving high efficiency in operation.
In general, in a gas turbine in which tip clearance is not controlled, tip clearance is minimized at a startup position, not at a position during rated operation. Thus, with the ACC system, an operating state with minimum tip clearance is set during rated operation by warming up stationary components that affect the tip clearance in a step before starting up the gas turbine. In other words, as illustrated in FIGS. 12A to 12D, the ACC system is a technique for minimizing clearance during rated operation to ensure the operating efficiency by increasing the clearance in advance by warming up the turbine stationary parts before starting up the gas turbine and adjusting the temperature of the turbine stationary parts during rated operation.
The operation of a gas turbine with the above-described ACC system can be broadly classified into the following five states.
(1) Immediately Before Startup
To apply the ACC system, the stationary components of the turbine stator blades are warmed up by letting a temperature-control medium (heating medium) flow therethrough, thus increasing the expansion to increase the clearance between stationary parts, such as a blade ring, and rotor blades, which are rotary parts.
(2) During Startup (while Increasing Load)
In the same manner as immediately before startup, the stationary components continue to be warmed up so that the clearance does not disappear during startup (so that the stationary parts and the rotary parts do not contact each other).
(3) During Rated Operation
The clearance between the stationary parts and the rotary parts is minimized by changing the conditions (temperature etc.,) of the temperature-control medium (heating medium) flowing through the stationary components.
(4) During Shut-Down (while Lowering Load)
In the same manner as immediately before startup, the stationary components continue to be warmed up so that the clearance does not disappear during shutdown (so that the stationary parts and the rotary parts do not contact each other).
(5) During Shut-Down
High-temperature gas remaining inside the gas turbine is exhausted outside the gas turbine to prevent cat back. Furthermore, the distribution of gas remaining inside the gas turbine is eliminated by letting the temperature-control medium (heating medium) flow through the stationary components to prevent cat back.
In the above-described ACC system, the clearance control method for the gas turbine is classified into the following three methods.
(1) Control Method Based on Changing the Conditions of the Cooling Medium Flowing Inside the Turbine Blades
This is a control method in which the temperature of the cooling medium flowing inside the turbine is changed by changing the cooling method for the cooling medium etc. (for example, changing from no cooling to air cooling or steam cooling), thereby adjusting the clearance by changing the amount of expansion of the turbine blades themselves, which requires a mechanism for changing the cooling method for the cooling medium.
(2) Control Method Based on Adjusting the Temperature of Stationary Components by Steam or Air
This is a method for controlling the clearance by letting steam etc. generated at an exhaust gas boiler flow through the stationary components after adjusting it with valves, etc.; in general, when air is used, the cycle efficiency decreases because the air is discarded to the gas path side without collecting it.
Furthermore, when steam is used, the startup time is long because operation in a simple cycle cannot be achieved, and boiler warm-up is required. Moreover, when steam is used, additional equipment is required, such as an auxiliary boiler for startup and steam piping from the exhaust gas boiler.
(3) Control Method Based on Moving Blades or Casing with a Mechanical Mechanism
This is a control method for adjusting the clearance by providing a mechanical mechanism, such as an actuator, and moving the blades and casing.
As related art of the above-described ACC system, compressed air is extracted and, after passing through a flow regulating valve, cools a segmented ring of the stationary components (for example, refer to PTL 1).
Furthermore, part of the steam used in the steam turbine is taken out and returned to the steam turbine system after adjustment by a valve and cooling the segmented ring (for example, refer to PTL 2).
In this way, the cat back problem has been noted when the gas turbine is stopped by the ACC system. Cat back is a phenomenon in which, while the gas turbine is stopped, the gas turbine warps due to a temperature difference. That is, since temperature layers are formed inside the gas turbine even after it has stopped because the inside of the gas turbine reaches a high temperature during operation, a temperature difference is generated between the upper part of the gas turbine (high temperature) and the lower part of the gas turbine (low temperature). As a result, the entire gas turbine warps in a stooping manner due to a difference in the amount of expansion generated between the upper part and lower part of the gas turbine.
As related art to prevent such cat back, a nozzle is provided on the upper part of a cylinder casing, and cooling air flows toward the upper part of the wall surfaces in the cylinder to decreases the upper and lower temperature difference (for example, refer to PTL 3).
Furthermore, there is one in which openings are formed in the cylinder lower part and the cylinder upper part to circulate air in the cylinder using a pump (for example, refer to PTL 4).